Bandpass filters are filters which selectively pass signals within a certain frequency range. Signals outside this bandwidth are attenuated so that they do not interfere with the desired signal. On the other hand, bandstop filters are filters which selectively attenuate signals within a certain frequency range. Signals outside this bandwith are passed by the filter.
The receive bandpass filter used in a transceiver must be capable not only of passing the full range of receive frequencies, but must also provide attenuation of the power at the transmit frequency during full duplex operation so that the receiver can operate when the transmitter is switched on. If the rejection is too low, then the transmit signal can enter the receiver as noise. Therefore the bandpass filter used in the receiver must be of a high enough order to reject the top transmit frequency, whereas a signal at the bottom receive frequency must be passed with a viable insertion loss.
Similarly, the transmit bandstop filter used in the transceiver must be capable of passing the full spectrum of transmit frequencies, while also providing sufficient attenuation of noise power in the receive band. The bandstop filter used in the transmitter must be of a high enough order to reject the lowest receive frequency, whereas a signal at the highest transmit frequency must be passed with viable insertion loss.
In cellular radio telephone systems the separation between adjacent edges of the receive and transmit bandwidths may be relatively close. For example in the ETACS cellular system in operation in the United Kingdom the transmission bandwidth is 872-905 MHz and the receiver bandwidth is 917-950 MHz. In this case the separation between adjacent edges of the receive and transmit bands is 12 MHz.
The spectral separation between the edges of the receive and transmit bands clearly impacts on the filtering requirements. When this separation is low, as in ETACS, higher order filters are generally required to meet the more demanding filtering requirements.
Higher order filters generally have more poles and hence a more complex physical and electrical configuration. Consequently higher order bandpass filters tend to be both large in size and costly to manufacture. The high cost is an obvious disadvantage. The large size means that higher order filters are not compatible with portable transceiver apparatus where miniaturization is desirable.
A further disadvantage of higher order bandpass filters is the associated insertion loss. That is to say, the greater the amount of filtering of the undesired signal, the higher is the loss to the desired signal.
The Applicant has recognised that an alternative solution to using higher order filters is to use a tunable filter. The stopband/passband of the filter may be varied electronically to coincide with the channel currently being received. The passband of the filter is selected to occur at a fixed level above the transmission frequency, for example 45 MHz in the case of ETACS. The tunability gives the advantage that the required filtering performance can be achieved with a lower order filter.
U.S. Pat. No. 4,835,499 discloses a voltage tunable combine filter having five poles. This bandpass filter comprises five parallel resonators which are coupled electromagnetically. Tuning diodes, used to vary the filter's passband, are coupled to one end of each resonator. The resonators are DC isolated at the opposite ends of the resonators, which ends are RF grounded. However, this combine configuration is relatively large in size (bearing in mind that the length of the individual transmission lines is in the order of a quarter of the wavelength of the received signal) and therefore not conducive for inclusion in compact portable transceiver apparatus. Furthermore it is noted that the filter configuration disclosed in this U.S. patent comprises five poles, but does not include any specific means for generating a transmission zero (i.e. rejection band or "notch"). Indeed it would not be possible to introduce such zero-generating (i.e. attenuating) means in the structure envisaged therein.
European patent application EP-A-0,364,931 discloses a bandpass filter having a tunable attenuation pole. The filter comprises a plurality of parallel dielectric resonators formed in a monolithic block of dielectric material. Adjustment electrodes associated with each resonator are provided on one face of the dielectric block. Either the pitch of the resonators or the configuration of the adjustment electrodes can be varied to cause inductive or capacitive overcoupling thereby tuning an attenuation pole of the dielectric filter to a selected frequency outside the passband. A disadvantage of this filter is that the actual physical configuration (i.e. resonator spacing or electrode pattern) has to be altered for different frequency requirements. In other words, individual filters are designed to give specific filtering characteristics and they are not readily re-tunable.
Our European patent application EP-A-0,441,500 relates to a transceiver for a cellular telephone in which a notch filter tuned to the transmit frequency is coupled to the output of the first amplifier in the receiver. The notch filter may be tunable to vary the location of the notch.